Matched filters as optimal linear filters for maximizing the signal to noise ratio (SNR) of filtered signal in stochastic additive noise, are widely used in signal detection, image processing, and telecommunications. A matched filter performs correlation on a known target signal with an unknown input signal to detect the presence of the target signal in the input signal, and maximizes the signal to noise ratio (SNR) of the output signals. Specifically, a matched filter generally convolves the input signals with a conjugated time-reversed version of the target signal (or a signal parallel to the target signal), and at the time the input signal is parallel to the target signal, the absolute value of the output signal must be a positive number. Therefore, a conventional signal processing system adopting a matched filter for determining whether a receiving signal is a target signal generally comprises at least a matched filter, an absolute value calculation device, and a comparing device for comparing the accumulated absolute value of a series of output values with a predetermined value, so as to determine the receiving signal is identical to the target signal while the accumulated absolute value exceeds the predetermined result.
FIG. 1 is a block diagram of a signal processing system 10 of the prior art. The system 10 comprises a sampling device 120, a matched filter 140, an absolute value calculation device 160 and a determination device 180. The matched filter 140 comprises delay units 142, multiplication units 144, conjugation units 146 and an accumulation unit 148.
The sampling device 120 receives an unknown input signal and samples it to generate sampled values r(n), wherein n is a predetermined positive integer number. A plurality of delay units 142 delays the sampled values r(n) to generate delayed sampled values r(n), r(n+1), . . . , and r(n+N−1), wherein N is also a predetermined integer number. A plurality of conjugation units 146 conjugate reference values R(n) to generate a plurality of conjugate reference values R*(n), R*(n+1), . . . , and R*(n+N−1), wherein R(n) are template signals parallel to the target signal. At that stage, the template signals are an ideal, distorted copy of the target signal. A plurality of multiplication units 144 multiply the delayed sampled values and the conjugate reference values one-to-one to generate r(n)R*(n), r(n+1)R*(n+1), and r(n+N−1)R*(n+N−1). The accumulation unit 148 accumulates r(n)R*(n), r(n+1)R*(n+1), . . . , and r(n+N−1)R*(n+N−1) to generate
      ∑          i      =      0              N      -      1        ⁢            r      ⁡              (                  n          +          i                )              ⁢                            R          *                ⁡                  (                      n            +            i                    )                    .      The absolute value calculation device 160 obtains the absolute value of
      ∑          i      =      0              N      -      1        ⁢            r      ⁡              (                  n          +          i                )              ⁢                  R        *            ⁡              (                  n          +          i                )            to generate accumulated absolute values x(k). The determination device 180 compares the accumulated absolute values x(k) with a threshold value to generate a determination result for determining whether the receiving signal is the target signal. However, such a system cannot eliminate the factor of the strength of the input signal, that is, the value of the accumulated absolute value x(k) is also related to the strength of the input signal. This infers the threshold value must vary according to the strength of the input signal, which is closely related to the circumstances and the way the system is implemented.
FIG. 2A and FIG. 2B are schematic diagrams of determining whether the receiving signal is the target signal of the prior art. In general, when determining whether the receiving signal is the target signal, comparing the result of the receiving signal undergoing the matched filter operation with the threshold value is performed. When the result of the receiving signal undergoing the matched filter operation is greater than the threshold value, the receiving signal is determined to be the target signal; otherwise, it is determined a non-target signal. When the period of the sampled values r(n) of the signal is 16, i.e. r(n)=r(n+16), the above N is set to be 16. When the receiving signal is the target signal, a larger matched filter result must be generated within one period, as shown in FIG. 2A. However, if the energy of the signal is too low, such as the signal received by the signal processing system 10 being interfered by noise, or the signal processing system 10 being too far away from the transmitting end, even the maximal matched filter result is no greater than the threshold value, such that the receiving signal cannot be determined correctly whether it is the target signal. On the contrary, when the energy of the signal is too high, such as the signal processing system 10 being too close to the transmitting end, or a constructive interference occurring, even if the matched filter results of the non-target signals and the reference value R(n) are not particularly large, the matched filter results may still exceed the threshold value, as shown in FIG. 2B, resulting in incorrectness in determining whether the receiving signal is the target signal.
A good signal processing apparatus must be capable of determining whether the receiving signal is the target signal despite the strength of the receiving signal. However, the prior art utilizes a fixed threshold value to determine different signals, which is inflexible and causes error easily. Hence, there is an urgent need for a signal processing apparatus and associated method for determining whether a receiving signal is a target signal more correctly.